In a legal system where Fourth Amendment protections seem to be eroding, there is hope yet.

The US Court of Appeals for the Third Circuit ruled in US v. Katzin that police need a warrant before attaching a GPS tracking device onto an individual's vehicle. In 2010, police had a strong suspicion that the Katzin brothers were involved in a number of robberies. To verify this suspicion, police affixed a GPS tracking device onto Harry Katzin's minivan to monitor the vehicle's movement. Shortly thereafter, the tracking device revealed the car was parked for an extended period of time next to a Rite Aid pharmacy. When the van began moving, police followed and stopped the vehicle. All three brothers were found inside along with a number of stolen items from the pharmacy.

The court emphasized that "a warrantless search is not rendered reasonable merely because probable cause existed that would have justified the issuance of a warrant." In other words, absent highly specialized circumstances, police officers cannot bypass the warrant requirement by later showing that probable cause existed at the time of action. The court dismissed the government's argument that requiring a warrant would improperly burden police work:

The government contends that requiring a warrant prior to GPS searches would "seriously impede the government's ability to investigate drug trafficking, terrorism, and other crimes." We fail to see how such a conclusory assertion suffices to except GPS searches from the requirements of the Fourth Amendment's Warrant Clause. Doubtless, we are aware of the dangers posed by terrorism and comparably reprehensible criminal activity. However, we would work a great disservice by permitting the word "terrorism" (in the absence of any other information or circumstance) to act as a skeleton key to the liberties guaranteed under the Constitution.

Prior to Katzin, in US v. Jones, upon suspecting that a nightclub owner was dealing narcotics, police planted a tracking device on the suspect's car and monitored the vehicle's movement for an entire month without a warrant. The Supreme Court held that using a GPS tracker constitutes a search under the Fourth Amendment. However, whether the search required police to obtain a warrant was unclear until now.

Nowadays, police can collect a vast amount of information without showing probable cause that an individual has likely committed a crime, and they can do it in a perfectly legal manner. Police have access to information that customers voluntarily give to corporations; information through online social networking sites such as Facebook, MySpace and Twitter (in fact, the government can get your tweets even after you delete them through a subpoena) and any files, photos or documents stored on a cloud. In essence, the police can create a legitimate narrative about who you are, what you do and who you associate with through piecing together fragments of your online presence.

You may be thinking why is the GPS decision important if the government has access to such a wealth of information anyway?

It's a matter of principle and autonomy. When it comes to voluntary information, an individual can theoretically choose not to have a cloud, abstain from social media and refuse to give corporations any information. Those choices would be perfectly within an individual's rights and would at least mitigate (not eradicate) the government being able to accumulate information about that individual. If we choose not to leave a digital footprint, the police should not be able to bypass the warrant requirement by collecting that information by simply tracking our movements.

The Katzin decision is an important step in establishing ascertainable limits and constraints to Fourth Amendment protections. Given the broad scope of GPS surveillance, warrantless searches have the potential to garner limitless information about individuals. Through a GPS tracker, the police can collect information about who your friends are, where you like to eat, what schools your children attend, your hobbies, which doctor you go to and much more. A warrant requirement provides oversight and curbs the danger of police stockpiling information.

As technology continues to evolve and become ever expansive, our Fourth Amendment rights are vulnerable to unwarranted government intrusion. In a world where "reasonable expectations of privacy" are constantly changing, how to balance privacy and technology is and will remain an important question. It is precisely the reach and access that the government has to information in the digital age which makes critical examination of privacy laws an important ongoing process.

Samar Warsi is a legal fellow at the Institute for Social Policy and Understanding. She holds a B.A. in Political Science from McMaster University and graduated with a J.D. from the Oklahoma City University School of Law. She is admitted to practice in the state bar of Texas.